A prime number is a whole number greater than 1 whose only divisors are 1 and itself. To test a number n, the checker tries dividing by every candidate up to the square root of n. If none divides evenly, n is prime; otherwise the divisors give its prime factorization.
Example: 97 has no divisor between 2 and 9 (its square root is about 9.8), so it is prime. 91 looks prime but equals 7 x 13.
No. Primes must have exactly two distinct divisors, and 1 has only one. That rule keeps factorization into primes unique.
Yes, and it is the only even prime number. Every other even number is divisible by 2.
25: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89 and 97.
See also: GCF and LCM calculator · random number generator · Roman numeral converter
More number tools: square root calculator.
Counting arrangements? See the factorial, permutation and combination calculator.